Optical time break detector

ABSTRACT

A method and apparatus for detecting the instant on explosive is detonated by way of an optical signal. A length of optical fiber has one end wrapped around or positioned near the explosive with the other end coupled to a remote recorder. The end of the fiber at the explosive has been cleaved so as to internally reflect an optical signal propagating therein. A beam of radiation is launched into the fiber at the record where it is reflected from the distal end and redirected back to the recorder where it is detected. Detonation of the explosive destroys the reflective end of the optical fiber causing the radiation to leak from the broken fiber end. The instant the recorder does not receive the reflected radiation is the instant of the explosive detonated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to seismic exploration and particularly to amethod and apparatus for determining the instant an explosive isdetonated.

2. Description of the Related Art

In seismic exploration using explosives as the seismic source, it isdesirable to know precisely, the instant of detonation. Currently, thetime of detonation, commonly referred to as the "time break" isdetermined by measuring the time the electrical continuity between thetwo wire leads of a blasting cap is broken. Closing the switch in thedetonator circuit causes a signal to be sent to the controller resultingfrom the brief flow of current through the circuit before the lead wiresin the blasting cap are broken. Plasma generated by the explosion of theblasting cap conducts current in the circuit after detonation, thus thesignal sent by the detonator to the time recorder is actually longerthan that actually representing the instant of detonation. The accuracyof that method is in the order of 0.1 millisecond (ms). With the adventof more modern and accurate geophysical equipment, it is desired todetermine the instant of detonation with an accuracy of one microsecondor better.

SUMMARY OF THE INVENTION

It is an object of this invention to determine the exact instant ablasting cap is detonated.

In accordance with an object of this invention, a length of disposableoptical fiber may be wrapped around or placed in close proximity to theblasting cap. One end of the optical fiber is cleaved and polished so asto cause complete internal reflection of a beam propagating therein. Theopposite end of the fiber has a three decibel coupler with one of thetwo ports of the coupler connected to a light source such as a laser andthe other port connected to a light detector. The light detector isoperably coupled to a controller which is interfaced with the detonatorcircuit of the blasting cap. The light source directs a coherent beam ofradiation into the optical fiber, which propagates to the end and istotally reflected back in the opposite direction where it is detected bythe light detector. When the detonator circuit is closed and theblasting cap is detonated, the force of the explosion will break theoptical fiber, interrupting the reflected optical signal. The instantthe light detector does not receive the reflected radiation issubstantially the instant of detonation which is recorded.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the benefits and advantages of my inventionmay be obtained from the appended detailed description and the drawings,wherein:

FIG. 1 is a general diagram of an explosive charge in a shot hole; and

FIG. 2 is a generalized schematic diagram of the apparatus of thisinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a generalized diagram of a seismic shot point 10 employing anoptical time-break detector 12 of the instant invention. The shot point10 includes a shot hole 14 having a predetermined depth d, such as twohundred feet below the surface of the earth 16, having a quantity ofexplosive 18, such as dynamite, disposed at the bottom thereof. Theexplosive 18 has at least one blasting cap (not shown in FIG. 1)embedded therein, coupled by wire leads 20 to a detonator 22 on thesurface 16. The optical time-break detector 12 includes a time-breakrecording unit 24, interfaced with the detonator 22, from which a lengthof optical fiber 26 extends into shot hole 14. The free end of opticalfiber 26 may be wrapped around the blasting cap or be within at leastsix inches thereof.

FIG. 2 is a generalized schematic diagram of the optical time-breakdetector 12. A light source 30 such as a light emitting diode (LED)within the recording unit 24 is in optical communication with opticalfiber 26 by way of a 3 decibel (dB) optical coupler 32 and optical-fibersegment 33 coupled near a first end of the fiber 26. The first end ofoptical fiber 26 is in optical communication with a light detector (LD)34 which is operably coupled to a controller 36 which in turn isoperably coupled to a recorder 38. Controller 36 may include amicroprocessor for internal timing. The controller 36 is poweredexternally by line PWR which also supplies power to the diodes 30 and 34and the recorder 38. The opposite or second end of optical fiber 26 maybe wrapped around, or located proximate to the blasting cap 40 which isembedded within the explosive 18 or the fiber may be wrapped around theexplosive charge itself. The second end of optical fiber 26 may becleaved and polished substantially perpendicular to the longitudinalaxis of the fiber 26. This creates an internal optically-reflective endsurface. The blasting cap 40 is in turn coupled by wire leads 20 to abattery 42 through blaster switch 44 in the detonator 22. The detonator22 is interfaced with the controller 36 by conductors 46.

In operation, when light source 30 is turned on, it generates a coherentbeam of radiation having a predetermined band width which is launchedinto optical fiber 26 via optical-fiber segment 33 and the 3 dB opticalcoupler 32. The radiation propagates down optical fiber 26 and isreflected in the opposite direction by the cleaved second end of thefiber at the blasting cap 40. The counter propagating radiation returnsto the first end of the fiber 26 where it is directed into the lightdetector 34 creating a logical one state of a control voltage in thecontroller 36 which may be recorded on recorder 38.

When blaster switch 44 is closed, cap 40 explodes, rupturing the end offiber 26, interrupting the reflected light beam. Absence of the lightbeam causes the recorded control voltage to drop to a logical zero. Theinstant of change is the time break or cap fire-time.

In another embodiment of this invention, a length of optical fiber mayhave a first end coupled to the light source such as the LED 30, and asecond end coupled to the light detector 34 forming an optical loop.Midway along the optical loop, between the LED 30 and light detector 34,the optical fiber may be wrapped around or proximate to the blasting cap40 similar to that described above. Detonation of the explosiveinterrupts the optical path of the radiation beam, resulting indetermination of the time of detonation.

For illustrative purposes, my invention has been described with acertain degree of specificity. Variations will occur to those skilled inthe art but which may be included within the scope and spirit of thisinvention which is limited only by the appended claims.

I claim:
 1. A method for detecting the instant an explosive isdetonated, comprising the steps of:(a) directing a coherent beam ofradiation through an optical waveguide having a portion proximate saidexplosive; (b) receiving said coherent beam of radiation from saidoptical waveguide at a recording unit; (c) causing said explosive todetonate so as to break said portion of said optical waveguide proximatethereto; and (d) recording the instant said coherent beam of radiationis not received by said recording unitl
 2. An apparatus for detectingthe instant an explosive is detonated, comprising:(a) an opticalwaveguide having a first and a second end and a predetermined portion ofthe optical waveguide proximate said explosive; (b) means for launchinga coherent beam of radiation into said optical waveguide; (c) means forreceiving said coherent beam of radiation from said optical waveguide;and (d) means for recording the instant said coherent beam of radiationis not received by said means for receiving when said explosive isdetonated.
 3. An apparatus for substantially determining an instant intime an explosive is detonated, comprising:(a) waveguide means having afirst and a second end, said first end having an internal opticallyreflective surface, said first end being disposed proximate saidexplosive; (b) means coupled to said second end, distant from saidexplosive for directing a beam of radiation into said waveguide means soas to direct said beam against said internal optically reflectivesurface at said first end; (c) means coupled to said second end of saidwaveguide means for receiving said beam of radiation reflected from saidfirst end; (d) means for detonating said explosive so as to rupture saidwaveguide means proximate thereto, so that rupture of said waveguidemeans interrupts said reflected beam of radiation; and (e) means forrecording the instant of interruption of said reflected beam ofradiation.
 4. An apparatus as recited in claim 2, further comprisingmeans a said first end of said optical waveguide for internallyreflecting said coherent beam of radiation incident thereon.
 5. Anapparatus as recited in claim 2, wherein said first end of said opticalwaveguide is cleaved and polished substantially perpendicular to alongitudinal axis of said optical waveguide.
 6. An apparatus as recitedin claim 2, further comprising:(a) said first end of said opticalwaveguide is coupled to said means for launching said coherent beam ofradiation therein; (b) said second end of said optical waveguide iscoupled to said means for receiving said coherent beam of radiation; and(c) said predetermined portion of said optical waveguide is midwaybetween said first and second ends.